tính nhanh 1/5+4/10+9/15+16/20+25/25+36/30+49/35+64/40+81/45 em cảm ơn ạ

1/5+4/10+9/15+16/20+25/25+36/30+49/35+64/40+81/45

= $\frac{1}{5}$ + $\frac{2}{5}$ + $\frac{3}{5}$ + $\frac{4}{5}$ + $\frac{5}{5}$ + $\frac{6}{5}$ + $\frac{7}{5}$ + $\frac{8}{5}$ + $\frac{9}{5}$ 

= ( $\frac{1}{5}$ + $\frac{9}{5}$ ) + ( $\frac{2}{5}$ + $\frac{8}{5}$ ) + ($\frac{3}{5}$ + $\frac{7}{5}$ ) + ($\frac{4}{5}$ + $\frac{6}{5}$ ) + $\frac{5}{5}$

= $\frac{10}{5}$ + $\frac{10}{5}$ + $\frac{10}{5}$ + $\frac{10}{5}$ + 1

= 2 + 2 + 2 + 2 + 1

= 9